These studies are directed toward understanding the marrow regulatory defect in dogs with cyclic hematopoiesis. In the first year of this grant methods have been established to study the in vitro growth of colony forming cells from canine peripheral blood and bone marrow. In the next phase of this work, the number and types of colonies of granulocytic and erythroid cells present at various phases of the cyclic hematopoietic process will be investigated. In addition, dose response studies will be conducted with marrow cells from normal and affected dogs and standard sources for stimulatory activity to determine if dogs affected with cyclic hematopoiesis have marrow cells which respond abnormally to stimulatory substances. We will also examine purified cell populations from dogs with cyclic hematopoiesis to determine if these cells contain abnormal amounts of stimulators or inhibitors of marrow cell proliferation. Treatment of the dogs to alter or eliminate the cyclic process will also be investigated.